1. Field of the Disclosure
The present disclosure relates to a Liquid Crystal Display (LCD) device, and more particularly, an LCD device and a method of manufacturing the same, which increases a margin between the channel width and length (W/L) of a thin film transistor having a multi-gate structure.
2. Discussion of the Related Art
With the advance of portable electronic devices such as mobile communication terminals and notebook computers, demands for Flat Panel Display (FPD) devices are increasing.
FPD devices, LCD devices, Plasma Display Panels (PDPs), Field Emission Display (FED) devices and Light Emitting Diode (LED) display devices continue to be researched and developed. Among the FPD devices, applications of LCD devices are being expanded because the LCD devices are easily manufactured, are easily driven, have high image quality and large screen sizes.
Touch screens that replace input devices such as mice or keyboards and allow a user to directly input information by finger, pen, or stylus are being applied to flat panel devices.
Touch screens are being applied in various fields, for example, mobile terminals for navigation, industrial terminals, notebook computers, financial automation equipment, game machines, portable terminals such as portable phones, MPEG Audio layer 3 (MP3) players, Personal Digital Assistants (PDAs), Portable Multimedia Players (PMPs), Play Station Portables (PSP), portable game machines and Digital Multimedia Broadcasting (DMB) receivers, and appliances such as refrigerators, microwave ovens and laundry machines. The application of the touch screens are being expanded because all users can easily manipulate the touch screens.
LCD devices with built-in touch screens are recently being developed to reduce the size of electronic equipment. Particularly, an in-cell type of LCD device that uses an existing element in the active structure such as a common electrode formed in a lower substrate as a touch sensing electrode is being developed.
FIG. 1 is a view illustrating the related art LCD device with built-in touch screen and driving method thereof. Referring to FIG. 1, the related art LCD device with built-in touch screen 10 includes a lower substrate 50 and an upper substrate 60 that are coupled with a liquid crystal layer (not shown) in between.
As an example of built-in touch screen operation, the pixel array 40 can also be used as a touch screen TS sensor. A small voltage may be applied to the pixel array 40 to create a uniform electrostatic field. When a conductor, such as a human finger or other object, touches the uncoated front surface, a capacitor Ctc is formed. A controller connected to the touch screen TS sensor can determine the location of the touch indirectly from the change in the capacitance as measured from the four corners of the touch screen TS sensor.
The upper substrate 60 includes a black matrix 62 that defines a pixel region so as to be in correspondence with each of a plurality of pixels, a red color filter 64R that is formed in a pixel defined by the black matrix 62, a green color filter 64G that is formed in a pixel defined by the black matrix 62, a blue color filter 64B that is formed in a pixel defined by the black matrix 62, and an overcoat layer 66 that is formed to cover the black matrix 62 and the color filters 64R, 64G and 64B to planarizes the upper substrate 60.
The lower substrate 50 includes a pixel array 40 that includes a plurality of pixels to drive the liquid crystal layer and for detecting the touch of a user's finger or a pen. Each of the pixels is defined by a data line and a gate line that intersect each other. A Thin Film Transistor (TFT) is formed in a region where the data line and the gate line intersect. Also, each of the pixels includes a common electrode and a pixel electrode.
FIG. 2 is a cross sectional view illustrating a lower substrate structure 50 of the related art LCD device with built-in touch screen. In FIG. 2, a lower substrate structure having a Fringe Field Switch (FFS) mode is illustrated. The FFS mode operation is described below.
Referring to FIG. 2, each pixel of the lower substrate 50 includes: a light shield layer 71 formed on a glass substrate; a buffer layer 51 formed on the light shield layer 71; an active layer 72 (i.e., a semiconductor layer) formed on the buffer layer 51; a gate insulation layer 52 formed on the active layer 72; a gate electrode 73 formed of a metal material on the gate insulation layer 52 to overlap with a portion of the active layer 72; an Inter Layer Dielectric (ILD) 53 formed on the gate electrode 73 to insulate the gate electrode 73 and the data electrode 74 (source/drain); and a data electrode 74 electrically connected to the active layer 72.
The gate insulation layer 52 and the ILD 53 are etched, and thus a first contact hole is formed to expose a partial region of the active layer 72. The data electrode 74 is formed by burying a metal material in the contact hole. Furthermore, the data electrode 74 is electrically connected to the pixel electrode 77 (pixel ITO).
Each pixel of the lower substrate 50 includes a first passivation layer 54 (PAS0) and a second passivation layer 55 (PAS1) that are sequentially formed to cover the gate electrode 73 and the data electrode 74. Each pixel of the lower substrate 50 also includes a common electrode 75 formed on the second passivation layer 55, and a conductive line 76 (3rd metal) that is formed on one side of the common electrode 75 and electrically connected to common electrodes of adjacent pixels. Also included are a third passivation layer 56 (PAS2) that is formed to cover the common electrode 75 and the conductive line 76, and a pixel electrode 77 that is formed to be electrically connected to the data electrode 74.
A thin film transistor being a switching device of an LCD device may be formed in a top gate structure or a bottom gate structure. When the thin film transistor is formed in the top gate structure, a backlight irradiates light on the active layer 72 through the substrate, as shown. Therefore, a light leakage current occurs in the active layer 72 image degradation such as crosstalk may arise. Crosstalk is an undesirable visual phenomenon resulting from unintended pixels turning on to image misinformation. The combination of residual gate voltage during the decay time after the gate is turned off plus photonic energy absorbed from the backlight unit may be enough to at least partially turn the TFT on when it is intended to be off.
To prevent such limitations, a metal layer, i.e., the light shield layer 71 for shielding light is disposed under the active layer 72. Therefore, light of the backlight is prevented from being irradiated on the active layer 72, and thus a leakage current is minimized.
The electron mobility property of amorphous silicon limits the operational speed and the geometric design rules of the TFT. To overcome such limitations, low temperature poly-silicon (LTPS) is being used as a material for forming the active elements (for example, TFT) of the lower substrate 50 because the electron mobility is about 100 times higher than a-Si. Even when LTPS is used as a material for forming the TFT of the lower substrate 50, as illustrated in FIG. 3, ten (10) masks corresponding to patterned layers are used in a manufacture process, and therefore, a plurality of detailed processes 155 (steps) are performed.
LTPS enables higher resolution display panels as compared to a-Si, and has excellent characteristic for TFT operations. However, LTPS requires the manufacture process to have more masks and detailed processes than a-Si because there are extra annealing steps. Therefore, the price competitiveness is limited and manufacturing efficiency is reduced.
As described above, with regard to the top gate structure, when forming the light shield layer 71 for preventing the light leakage current of the active layer 72, a separate mask for forming the light shield layer 71 is used. Therefore, the manufacturing cost increases because extra steps to process the separate mask are performed, causing a reduction in productivity.
FIG. 4 is a view illustrating a multi-gate structure of a related art LCD device.
Referring to FIG. 4, to improve limitations where a manufacture method increases due to the use of LTPS, proposed has been a method that removes the light shield layer 71 and form a TFT in a multi-gate structure for reducing a light leakage current. The shape of a pixel is very narrow in a high resolution LCD device. Therefore, when forming a gate pattern 80, an interval margin, the difference between geometric features of the TFT, between lines is insufficient.
Accordingly, when forming a photoresist (PR) pattern 90 for forming the gate pattern 80, the PR pattern 90 is distorted due to process limitations. Due to the PR distortion, a corner portion of the PR pattern 90 is rounded rather than sharp. When a gate pattern is formed with the PR pattern 90 having a rounded corner portion, distortion occurs in the gate pattern. This distortion causes the margin between the channel width and length (W/L) of a TFT to be reduced. Due to the reduction in the margin, the driving performance of an LCD device is degraded because there are differences between the TFT design and what is actually produced.